In the following, some examples of possible applications for measuring or determining a degree of linearity of an amplifier will be described.
Amplifiers are widely used in nearly any kind of electronic device. Depending on the intended use of the amplifier, a suitable amplifier design can be selected from a wide variety of amplifier designs developed and implemented to date. For example, an amplifier may be used in an output stage in order to drive some kind of transducer such as a loudspeaker, an antenna, a mechanical actuator, or a light source. An amplifier in an output stage is typically referred to as a power amplifier (PA) and is classified as class A, B, AB, and C for analog designs, and class D and E for switching design. These classes differ, among others, with respect to a degree of linearity that can be achieved and also with respect to their efficiency. For example, class A amplifiers are typically more linear than other types, but are very inefficient. A class C amplifier on the other hand offers the advantage of high efficiency, but a disadvantage is high distortion, i.e. the degree of linearity is relatively poor.
One field of technology in which PAs are widely used is the field of mobile telecommunication to drive the antennas of base transceiver stations or mobile devices. In both cases, power efficiency is the concern of the design process of the PA's. In the case of a PA used in a base transceiver station, the PAs make up a major part of the total power consumption, and thus often the operating costs of a base transceiver station. In a mobile device, improving the efficiency of the PA is desirable in order to achieve a longer battery life and/or standby time.
As mentioned above, improving the efficiency of a power amplifier typically results in a degradation of the linearity. Modern telecommunication standards, however, make relatively high demands on the linearity of devices operating under these standards in order to reduce inter-modulation. An excessive inter-modulation typically results in the output signal of the amplifier having a spectrum that contains frequencies which are not in the input signal, i.e. the spectrum is broadened. Typically, this effect is undesired because it results in the generation of jamming signals for other telecommunication systems and a waste of bandwidth.
One way of saving current in a transmitter for mobile communication is to “reduce the bias” of the PA to the minimum allowing the system to operate within the constraints dictated by the communication standard. All modulation types except GMSK (Gaussian Minimum Shift Keying) need a certain linearity to be guaranteed, therefore in an adaptive system capable of dynamically reducing the bias to control the consumption of DC-power, it is desirable to provide an effective and easy way of assessing the degree of linearity of the output signal.
Biasing in electronics is the method of establishing predetermined voltages and/or currents at various points of an electronic circuit to set an appropriate operating point. Linear circuits (or circuits operating in an approximately linear manner) involving transistors typically require specific DC voltages and currents to operate correctly, which can be achieved using a bias circuit. The bias voltage applied to a transistor in an electronic amplifier typically allows the transistor to operate in a particular region of its trans-conductance curve. Thus, the operating point and the corresponding bias voltage and/or current may preferably be selected so that the constraints suggested or even dictated by the communication standard are met at a relatively low level of biasing. The operating point at a corresponding bias setting may be implemented in a fixed, non-variable manner. The behavior of the amplifier varies, however, as a function of different variables such as temperature, power amplifier mismatch, level of output power, bias, and power amplifier specimen. At present a preliminary investigation of the signal-quality as a function of different variables is performed at laboratory by precision instruments. A fixed table for the best bias settings is then generated and used in the firmware of the products adopting that certain family of power-amplifiers.